Curable, or thermosettable, coating compositions are widely used in the coatings art, particularly for topcoats in the automotive and industrial coatings industry. Basecoat-clearcoat composite coatings are topcoats that offer exceptional gloss, depth of color, distinctness of image, or special metallic effects. The automotive industry has made extensive use of basecoat-clearcoat composite coatings for automotive body panels. Single layer topcoats and the clearcoats of color plus clear composite coatings usually require an extremely high degree of clarity and gloss to achieve the desired visual effect. Furthermore, they must maintain the clarity and gloss over long periods of time in the fact of environmental challenges.
Clearcoat coating compositions used as the outermost automotive coating are subject to damage caused by numerous elements. These elements include environmental fall out, exposure to ultraviolet radiation from sunlight, exposure to high relative humidity at high temperature, and defects made by impacts of small, hard objects resulting in chipping. “Environmental etch” is a term applied to a kind of exposure degradation that is characterized by spots or marks on or in the finish on the coating that often cannot be rubbed out.
Resins for coating compositions are selected by the formulator to provide desirable properties associated with the structure of the resin. Because most resins require curing before development of optimum coating properties, it is usually necessary for the formulator to add a crosslinking agent or resin to the coating composition. The crosslinking resin, in general, contains reactive functional groups that react with the functional groups of the coating resin. In many cases, it is advantageous to provide crosslinking resins that are compatible with the coating resin and that have as high a functionality as possible. Such crosslinkers can be used in relatively small amounts without detracting from the coating properties of the composition.
Coating compositions based on carbamate-functional resins and aminoplast cross-linking agents are desirable as automotive topcoats because they have excellent durability, hardness, gloss, appearance, and resistance to environmental etch. As such, they are highly suitable for use as a clearcoat layer applied over a basecoat layer in a color plus clear composite coating. Especially for hardness and etch, it is desirable to provide coating compositions with a suitable level of crosslinkable sites. Futhermore, molecules having hydroxyl functionality are generally helpful in promoting intercoat adhesion.
It would be desirable to improve the crosslinking density of such compositions by adding a resin having hydroxyl or carbamate functionality and a relatively low equivalent weight. It would be further desirable to provide such a crosslinking resin by reaction of a hydroxyl functional amine with an isocyanate under mild reaction conditions. This invention provides such a solution.